Transmission circuit



-July15, 1941. I L. A. THOMPSON 2,249,502

TRANSMISSION CIRCUIT Filed Oct. 2, 1939 IOO OI FL I F/EE I b mvrva 1% $3 Box P/c-ruee 4 1 2 TEAMS/VI;

EQUIPMENT PIC TUR E 'rknusM 155/014 EQUIPMENT P! c TURE TRA NSM EQUIPMENT INVENTOR. LOU/5 AJHOMPSON 23.5 F g.4 2 7M ATTORNEY 5.

Patented July 15, 1941 TRANSMISSION CIRCUIT Louis A. Thompson, Lakewood, Ohio, assignor to Acme Newspictures, Inc., l\l ew York, I, a

corporation of Delaware Application October 2, 1939, Serial No. 297,435

4 Claims.

This invention relates to transmission circuits,

and more particularly to a circuitnetwork for interconnecting two or more stations'for the fac simile transmission and reception.

Although the'invention about to be described is particularly adapted to facsimile transmission, it will be obvious that the same may be useful in connection with radio broadcasting network, telephone'networks or networks used in wire com munication'where the various stations are interconnected by land lines.

In f-acsimile transmission it is .customary to send a picture from one focal point to a number of' different stations simultaneously. That is, a

picture may be transmitted originating in 'Cleveland and sent to New York, Detroit, Chicago, and St. Louis simultaneously. Or as frequently occurs, the picture may be transmitted from an east coast station 'such as New York to Cleveland where it is not only reproduced in Cleveland, but simultaneouslydistributed or relayed to St. Louis, Detroit, Chicago and other points west or south. The' lines connecting these various points are usually the regular telephone lines used for long distance service; Because of the large area and long distances covered, some trouble has been experienced in picture quality because line disturbances, due to storms, static, and operator personnelmay be originated in any of the lines connected to the outlying stations which disturbance is transmitted to the control station and may be reflected into the other stations. The effect of such disturbances on picture quality is well known, and results in spots or so-called rain or streaks which prevent accurate or good reproduction.

By the system of the present invention, although disturbances may originate in any of the outlying lines or stations, such disturbances are reduced to such an extent as to be of no consequence, if not in many cases entirely eliminated.

Still other advantages of the. system'and the invention itself, will become more apparent from the following description of an embodiment thereof which description'is illustrated by the accompanying diagrams and forms a part of this specification.-

In the drawing:

Fig.1;isa schematic view illustrating the circuit network of my invention as used in connection with picture transmission equipment, the particular apparatus being shown as arranged for the pro-picture sending period during which the apparatus is being lined up for receiving the picture} a 2 is-a similar view showing the arrangement during the period when the picture is being =-received-. from a remote station reproduced at the reception point and relayedto other outlying stations; r

Fig. 3 is a similarschematio View showing the arrangement for transmitting from the focal station to the outlying stations;

Fig; 4 is a diagrammatic view illustrating the efiect of the coupling network as applied to outgoings-ignals; and 1 Fig. 5 is a similar-view showing the effect on incoming signals. p

The apparatus has been shown in the block diagrams as comprising a number of separate elements orunits. It should be understood,

however, that" this is merely forthe purpose'of explanation-and that the units could be in single or multiple housings. Briefly the apparatus includes picture transmission equipment connected by a control box or switch to an amplifier and to the line coupling network which network includes -a transformer. Suitable power supply, not shown, is provided. A monitor and a; transmitter are also provided, being useful in the aligning operation and for monitoring ones own speech as well as hearing the speech'irom one of the outlying'points, and monitoring the pic-f ture signal. The amplifier may comprise several stages, two'stages being shown for the purput stage of the amplifier which may be, and v A plurality l preferably is, ofthe push-pull type. of secondaries 2 to 6 inclusive are provided for the coupling transformer, each of the secondaries being-connected to the outgoing .lines A Y to E inclusive. The incoming lines are repre-' sented by the dotted jimpedances a to e inclu sive connected to A to'E respectively. Each of the linesis grounded and each ungrounded line has a resistance (12 to l6 inclusive) inserted therein. A

Referring particularlytoFig. 1 which, as previously stated, shows the apparatus as arranged for lining up the various stations preparatory to receiving and/or relayingja pictureirom say s'ta-j 3" tionfiA, to B etc;- Duringv this period the picture' reproducing machine is shorted out by grounding the same as indicated at the control. box. This keeps speech out of the picture equipment. The first input stage of the amplifierisalSoShorted but in the control box. The

transmitter T is connected into one of the end stages of the amplifier. The diagram shows only two stages, but it is obvious that more stages could be used. The lines I and HH are connected to station A, by bridging at the control box, in order that station A may hear the operators voice and to provide means for the operator to monitor his own speech as well as hear As reply. The speech from the transmitter T appears in all the secondaries 2 to 6 inclusive and therefore is transmitted to the outlying stations A to E inclusive. With this arrangement all of the preliminary work prior to the actual relaying or sending of the picture is arranged, assuming that the picture is bein received from station A as above mentioned. Stations B to E inclusive hear the one side of the conversation originating at the relaying station and thus know when the arrangements have been made and when station A is ready to start the transmission. This same situation prevails if the picture was being transmitted from the focal point to all five stations except that in that instance it would be unnecessary to hold conversation with station A. I

All arrangements having been made, the controls in the control box are shifted, as best shown in Fig. 2, to eliminate the transmitter from the circuit (the signal to go ahead having been given) station A starts the transmission and the picture signals come in over the line A into the control box where it is connected to the local picture transmission equipment, the input to the amplifier is also bridged in the control box across the lines I00 and NH, the line IOI being terminated in a 600 ohm resistance 1' which is grounded to complete the circuit back to 2. The amplifier receives and amplifies the signal picked up from station A and feeds the amplified signals at zero level through the transformer back to stations B to E inclusive. Monitoring of the signal can be done with the monitor and the proper signal level maintained by the volume control VC on the amplifier in conjunction with the volume indicator Vl.

When it is desired to send a picture originating at the focal point, as best shown in Fig. 3, the lines I00 and IOI from station A are bridged in the control box as in Fig. 1, the input to the amplifier is also shorted by grounding the same, and the picture transmitter is coupled through the control box to the intermediate, or last stage of the amplifier. The level of the picture transmission equipment is monitored by the monitor and maintained at the desired level by the volume control in conjunction with the volume indicator VZ. Y

As previously mentioned, it is desirable to elimi nate interference which may occur from the out lying stations and is reflected into the other outgoing lines and may affect the quality of the picture. It is with particular reference to this;v interference elimination that the output coupling circuit is designed. The standard telephone line impedance is 600- ohms and this is represented at stations A to E by the dotted resistance: representations a to e respectively.

In the representation shown in Fig. 1 line inthe telephone line impedance of 600 ohms by inserting impedances in the form of resistance l2 to IE5 inclusive in each line in series with the secondaries 2 to 6 respectively, each impedance having a value of 590 ohms. The resistor I2 is of a lower value than the other corresponding resistors to allow for the impedance of the monitor. The combined impedance of the monitor and resistor I2 is approximately that of the corresponding resistors l3, l4, l5, Hi. This provides a total of 600 ohms impedance for each of the lines, thus effecting an impedance match with the telephone lines.

As best shown in Fig. 4, considering each secondary from the output standpoint, we have a 10 ohm secondary looking into 1190 ohms of impedance, slightly more than one-half of which is impedance b of the telephone line of station B. With this arrangement, when a 1.6 v. signal is supplied to the secondary, approximately .8 v. is placed on the line, this being a reduction of one-half and the volume of the telephone standard zero level.

This same condition exists for all of the stations. As to incoming interference, however, a different condition exists and this is best shown in Fig. 5. In this instance, we have the line B, the impedance of which is 600 ohms indicated at b looking into 600 ohms of impedance represented by the resistance i3 and the secondary 3. The condition, however, is radically different from that first described since only 10 ohms of the coupling network impedance is actually coupling the line' into the transformer. There is, therefore, a reduction in any interference coming in on the line of approximately 60 to 1. Incoming signals do not ordinarily exceed, under normal conditions, .8 v. The voltage, therefore, present in the secondary 3 is .8/60 or .0133 v. Hence, the maximum interference voltage existing in any secondary from an outgoing line never exceeds, .0133 v. Considering this voltage as being transferred from one secondary to another, there is a further voltage reduction due to the transmission characteristics described for Fig. 4. There, as previously stated, a secondary 4 looks into the divided impedance of substantially 1200 ohms represented by Hi and line impedance c only one-half of which is connected to the line. There is, therefore, a still further reduction in voltage of 2 to 1, reducing the voltage which gets into the line to .00666 v.

This voltage is so small that it has no effect I upon the signal being relayed or transmitted. Since this condition applies to all of the outlying stations'in the case of Fig. 3, nothing occurring in any of the outlying stations Will affect the picture being transmitted or relayed to the other stations- In the case of Fig. 2, nothing occurring in stations B tofE inclusive can affect the picmission lines including a transformer having an input and an output, the output of said transformer including a plurality of secondaries, each adapted for connection to a line, a matching im pedance disposed in series with each secondary and the line and of a high value relative to the secondary to provide a low impedance secondary looking into a high impedance only one-half of which is the line, and a high impedance line looking into a matched network including a low impedance secondary and a matching secondary impedance to provide a large voltage reduction from the line into the secondary.

2. A circuit of the class described in combination with a coupling transformer including a primary and a plurality of low impedance secondaries, means to reduce intercapacitive and inductive disturbances between the output circuits for said secondaries including a high impedance for each of the secondaries in series therewith and in combination with the secondaries, matching the impedance of each line to provide a de-' sired voltage transfer from the output secondary into the line and a decrease in voltage transfer from the line into the secondary.

3. A circuit of the class described, a transformer consisting of a primary and a plurality of secondaries, said secondaries being of low impedance with respect to the primary and to circuits to be fed, a matching network inserted between each low impedance secondary and the circuit fed to provide a match between the circuit fed and the secondary and to provide an impedance ratio of the circuit fed relatively large to that of said secondary to cause interference picked up in the secondary from the circuit fed to be very small. r

4. A circuit of the class described, .a transformer consisting of a primary and a plurality of secondaries, said secondaries being of low imped-V ance with respect to circuits to be fed, matching networks inserted between each low impedance secondary and the circuit fed to provide an impedance match therebetween, and a high impedance ratio between the circuit fed and said secondary. I

LOUIS A. THOMPSON. 

